Chemical Pressure Effect Research Articles

Superconducting Properties in Y-doped Semimetallic Bi3O2S3 Superconductors

We report the superconducting properties of Y-doped semimetallic Bi3O2S3 superconductors. From electrical resistivity measurements, the Tc,zero of the optimally Y-doped Bi3O2S3 was higher than that of pure Bi3O2S3. This Y content dependence of the superconducting properties in Bi3O2S3 is explained by the chemical pressure effect.

Evolution of Anisotropic Displacement Parameters and Superconductivity with Chemical Pressure in BiS2-Based REO0.5F0.5BiS2 (RE = La, Ce, Pr, and Nd)

In order to understand the mechanisms behind the emergence of superconductivity by the chemical pressure effect in REO0.5F0.5BiS2 (RE = La, Ce, Pr, and Nd), where bulk superconductivity is induced by the substitutions with a smaller-radius RE, we performed synchrotron powder X-ray diffraction, and analyzed the crystal structure and anisotropic displacement parameters. With the decrease of the RE3+ ionic radius, the in-plane disorder of the S1 sites significantly decreased, very similar to the trend observed in the Se-substituted systems: LaO0.5F0.5BiS2-xSex and Eu0.5La0.5FBiS2-xSex. Therefore, the emergence of bulk superconductivity upon the suppression of the in-plane disorder at the chalcogen sites is a universal scenario for the BiCh2-based superconductors. In addition, we indicated that the amplitude of vibration along the c-axis of the in-plane chalcogen sites may be related to the Tc in the BiCh2-based superconductors.

Effect of chemical pressure on the crystal electric field states of erbium pyrochlore magnets

We have carried out a systematic study of the crystal electric field excitations in the family of cubic pyrochlores Er$_2B_2$O$_7$, with $B=$~Ti, Ge, Pt, and Sn, using neutron spectroscopy. All members of this family are magnetic insulators based on 4$f^{11}$ Er$^{3+}$ and non-magnetic $B^{4+}$. At sufficiently low temperatures, long-range antiferromagnetic order is observed in each of these Er$_2B_2$O$_7$ pyrochlores. Our inelastic neutron scattering measurements probe the transitions from the ground state doublet to excited crystal electric field states belonging to the $J=15/2$ Hund's rules manifold. This allows us to quantitatively determine the energy eigenvalues and eigenfunctions of these $(2J+1)=16$ states across the Er$_2B_2$O$_7$ series. The different ionic sizes associated with different non-magnetic $B^{4+}$ cations correspond to positive or negative chemical pressure, depending on the relative contraction or expansion of the crystal lattice, which gives rise to slightly different local environments at the Er$^{3+}$ site. Our results show that the $g$-tensor components, which characterize the anisotropy of the ground state doublet eigenfunctions, are XY-like for all four members of the Er$_2B_2$O$_7$ series. However, the XY anisotropy is much stronger for Er$_2$Pt$_2$O$_7$ and Er$_2$Sn$_2$O$_7$ ($\frac{g_{\perp}}{g_z} > 25$), than for Er$_2$Ge$_2$O$_7$ and Er$_2$Ti$_2$O$_7$ ($\frac{g_{\perp}}{g_z} < 4$). The variation in the nature of the XY-anisotropy in these systems correlates strongly with their ground states, as Er$_2$Ge$_2$O$_7$ and Er$_2$Ti$_2$O$_7$ order into $\Gamma_5$ magnetic structures, while Er$_2$Pt$_2$O$_7$ and Er$_2$Sn$_2$O$_7$ order in the $\Gamma_7$ Palmer-Chalker structure.

Towards operando computational modeling in heterogeneous catalysis.

An increased synergy between experimental and theoretical investigations in heterogeneous catalysis has become apparent during the last decade. Experimental work has extended from ultra-high vacuum and low temperature towards operando conditions. These developments have motivated the computational community to move from standard descriptive computational models, based on inspection of the potential energy surface at 0 K and low reactant concentrations (0 K/UHV model), to more realistic conditions. The transition from 0 K/UHV to operando models has been backed by significant developments in computer hardware and software over the past few decades. New methodological developments, designed to overcome part of the gap between 0 K/UHV and operando conditions, include (i) global optimization techniques, (ii) ab initio constrained thermodynamics, (iii) biased molecular dynamics, (iv) microkinetic models of reaction networks and (v) machine learning approaches. The importance of the transition is highlighted by discussing how the molecular level picture of catalytic sites and the associated reaction mechanisms changes when the chemical environment, pressure and temperature effects are correctly accounted for in molecular simulations. It is the purpose of this review to discuss each method on an equal footing, and to draw connections between methods, particularly where they may be applied in combination.

Effect of chemical pressure on competition and cooperation between polar and antiferrodistortive distortions in sodium niobate

We present results obtained from a combination of dielectric and x-ray diffraction measurements for compositional design of $(1\ensuremath{-}x)\phantom{\rule{0.28em}{0ex}}\mathrm{NaNb}{\mathrm{O}}_{3}\ensuremath{-}x\mathrm{BaTi}{\mathrm{O}}_{3}\phantom{\rule{0.28em}{0ex}}(\mathrm{NNBT}x)$, which can induce interferroelectric phase transitions. Anomalies are observed in dielectric measurements performed for various compositions at 300 K, as well as at different temperatures for NNBT03. We observed the appearance(disappearance) of the superlattice reflections along with change in the intensities of the main perovskite peaks in the powder x-ray diffraction data, which provide clear evidences for structural phase transitions with composition and temperature. We found that increasing the concentration of $\mathrm{BaTi}{\mathrm{O}}_{3}$ leads to the suppression of out-of-phase rotation of octahedra and an increment in tetragonality ($c/a$ ratio), which promotes the polar mode at room temperature. The temperature-dependent powder diffraction study shows that the ferroelectric rhombohedral phase of pure sodium niobate gets suppressed for the composition $x=0.03$, and the monoclinic phase $Cc$ gets stabilized at low temperature. The monoclinic phase is believed to provide for a flexible polarization rotation and is considered to be directly linked to the high-performance piezoelectricity in materials due to presence of more easy axes for spontaneous polarizations than the rhombohedral phase.

Collapsed tetragonal phase transition in LaRu2P2

The structural properties of LaRu$_2$P$_2$ under external pressure have been studied up to 14 GPa, employing high-energy x-ray diffraction in a diamond-anvil pressure cell. At ambient conditions, LaRu$_2$P$_2$ (I4/mmm) has a tetragonal structure with a bulk modulus of $B=105(2)$ GPa and exhibits superconductivity at $T_c= 4.1$ K. With the application of pressure, LaRu$_2$P$_2$ undergoes a phase transition to a collapsed tetragonal (cT) state with a bulk modulus of $B=175(5)$ GPa. At the transition, the c-lattice parameter exhibits a sharp decrease with a concurrent increase of the a-lattice parameter. The cT phase transition in LaRu$_2$P$_2$ is consistent with a second order transition, and was found to be temperature dependent, increasing from $P=3.9(3)$ GPa at 160 K to $P=4.6(3)$ GPa at 300 K. In total, our data are consistent with the cT transition being near, but slightly above 2 GPa at 5 K. Finally, we compare the effect of physical and chemical pressure in the RRu$_2$P$_2$ (R = Y, La-Er, Yb) isostructural series of compounds and find them to be analogous.

Electronic structures of UX3 ( X=Al , Ga, and In) studied by photoelectron spectroscopy

The electronic structures of UX$_3$ (X=Al, Ga, and In) were studied by photoelectron spectroscopy to understand the relationship between their electronic structures and magnetic properties. The band structures and Fermi surfaces of UAl$_3$ and UGa$_3$ were revealed experimentally by angle-resolved photoelectron spectroscopy (ARPES), and they were compared with the result of band-structure calculations. The topologies of the Fermi surfaces and the band structures of UAl$_3$ and UGa$_3$ were explained reasonably well by the calculation, although bands near the Fermi level ($E_\mathrm{F}$) were renormalized owing to the finite electron correlation effect. The topologies of the Fermi surfaces of UAl$_3$ and UGa$_3$ are very similar to each other, except for some minor differences. Such minor differences in their Fermi surface or electron correlation effect might take an essential role in their different magnetic properties. No significant changes were observed between the ARPES spectra of UGa$_3$ in the paramagnetic and antiferromagnetic phases, suggesting that UGa$_3$ is an itinerant weak antiferromagnet. The effect of chemical pressure on the electronic structures of UX$_3$ compounds was also studied by utilizing the smaller lattice constants of UAl$_3$ and UGa$_3$ than that of UIn$_3$. The valence band spectrum of UIn$_3$ is accompanied by a satellite-like structure on the high-binding-energy side. The core-level spectrum of UIn$_3$ is also qualitatively different from those of UAl$_3$ and UGa$_3$. These findings suggest that the U~$5f$ states in UIn$_3$ are more localized than those in UAl$_3$ and UGa$_3$.

Effect of Zn-site substitution with Ga on non-Fermi liquid behavior in PrIr2Zn20

PrIr2Zn20 exhibits an antiferroquadrupolar (AFQ) order at TQ = 0.11K, above which temperature the electrical resistivity ρ(T) shows an upward curvature and the magnetic specific heat divided by temperature Cm/T follows − lnT dependence. The non-Fermi Liquid (NFL) behaviors have suggested formation of a quadrupole Kondo lattice. In the present work, we have studied the effect of Ga substitution for Zn on the NFL behavior by the measurements of the specific heat C, the magnetic susceptibility χ, and ρ of PrIr2Zn20-xGax (0 ≤ x ≤ 0.25). With increasing x, the characteristic temperature T0 defined as the temperature where the magnetic entropy Sm reaches (3/4)Rln2 is increased by a factor of 3.5. Similarly, another characteristic temperature TR below which ρ(T) starts decreasing with the upward curvature increases with x by a factor of 1.2. The increments of both T0 and TR may be attributed to the possible split of the Γ3 doublet by symmetry lowering of the Pr sites. Otherwise, the quadrupole Kondo lattice would be stabilized by the enhanced c-f hybridization due to the increment of the 4p electronic density and/or the chemical pressure effect.

Effect of chemical pressure on the electronic phase transition in Ca1−xSrxMn7O12 films

We demonstrate how chemical pressure affects the structural and electronic phase transitions of the quadruple perovskite CaMn7O12 by Sr doping, a compound that exhibits a charge-ordering transition above room temperature making it a candidate for oxide electronics. We have synthesized Ca1−xSrxMn7O12 (0 ≤ x ≤ 0.6) thin films by oxide molecular beam epitaxy on (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 (LSAT) substrates. The substitution of Sr for Ca results in a linear expansion of the lattice, as revealed by X-ray diffraction. Temperature-dependent resistivity and X-ray diffraction measurements are used to demonstrate that the coupled charge-ordering and structural phase transitions can be tuned with Sr doping. An increase in Sr concentration acts to decrease the phase transition temperature (T*) from 426 K at x = 0 to 385 K at x = 0.6. The presence of a tunable electronic phase transition, above room temperature, points to the potential applicability of Ca1−xSrxMn7O12 in sensors or oxide electronics, for example, via charge doping.

Bulk Superconductivity Induced by Se Substitution in BiCh2-Based Layered Compounds Eu0.5Ce0.5FBiS2−xSex

We report the effect of Se substitution on the crystal structure and superconductivity of BiCh2-based (Ch: S, Se) layered compounds Eu0.5Ce0.5FBiS2-xSex (x = 0-1). Crystal structure analysis showed that both lattice constants, a and c, increased with increasing x, which is different from the related La-doped system, such as Eu0.5La0.5FBiS2-xSex. This is due to Se substitution for both in-plane and out-of-plane Ch sites in the present Ce-doped system. Zero resistivity was observed for x = 0.2-1 above 2 K. Superconducting properties of Eu0.5Ce0.5FBiS2-xSex was investigated by magnetic susceptibility, and the highest superconducting transition temperature of 3.5 K was obtained for x = 0.6 with a large shielding volume fraction. The emergence of bulk superconductivity and metallic conductivity can be qualitatively described in terms of increased in-plane chemical pressure effect. A magnetic anomaly below 8 K, probably because of ferromagnetic order of magnetic moment of Ce3+ ions, coexists with bulk superconductivity in BiCh2 layer. Since the effect of Se substitution on the magnetic transition temperature is ignorable, we suggest that the coupling between the magnetic order at the (Eu,Ce)F layer and superconductivity at the Bi(S,Se)2 layer is weak.

Effect of chemical and hydrostatic pressure on electronic structure of BiPd2O4: A first-principles study

We examine the structural and electronic properties of BiPd2O4 with the use of density functional theory. We show that the generalized gradient approximation is incapable to describe correctly crystallographic and electronic ground state of BiPd2O4. The Hubbard U correction is necessary to obtain results that agree with experimental data. We investigated the effect of hydrostatic pressure and Pt-doping on BiPd2O4. We found that BiPd2O4 undergoes a pressure-induced phase transition from triclinic (space group P-1) to tetragonal (I41/a) at 3 GPa. Pt-substitution in planar coordination lead to the Jahn-Teller distortion in octahedral coordination and, as a results, the small magnetic moment appears at the palladium atom. We calculated the elastic and vibrational properties of the hypothetical BiPt2O4. Our first-principles calculations suggest that this compound is stable.

Dome of magnetic order inside the nematic phase of sulfur-substituted FeSe under pressure

The pressure dependence of the structural, magnetic and superconducting transitions and of the superconducting upper critical field were studied in sulfur-substituted Fe(Se$_{1-x}$S$_{x}$). Resistance measurements were performed on single crystals with three substitution levels ($x$=0.043, 0.096, 0.12) under hydrostatic pressures up to 1.8 GPa and in magnetic fields up to 9 T, and compared to data on pure FeSe. Our results illustrate the effects of chemical and physical pressure on Fe(Se$_{1-x}$S$_{x}$). On increasing sulfur content, magnetic order in the low-pressure range is strongly suppressed to a small dome-like region in the phase diagrams. However, $T_s$ is much less suppressed by sulfur substitution and $T_c$ of Fe(Se$_{1-x}$S$_{x}$) exhibits similar non-monotonic pressure dependence with a local maximum and a local minimum present in the low pressure range for all $x$. The local maximum in $T_c$ coincides with the emergence of the magnetic order above $T_c$. At this pressure the slope of the upper critical field decreases abruptly. The minimum of $T_c$ correlates with a broad maximum of the upper critical field slope normalized by $T_c$.

Chemical pressure effect in magnetic frustrated pyrochlore Nd2Pb2O7: A crystal-field analysis

Variation of chemical pressure at R-site due to substitution of nonmagnetic cation of varying size at the M-site makes a fine tuning between the crystal-field and molecular field to adopt exotic ground states in the frustrated magnetic R2M2O7 pyrochlore structures. Presence of larger cation at M-site increases the lattice parameter or nearest-neighbor bond distance between magnetic R-spins, and causes subtle changes to the local oxygen environment surrounding each R-ion, thereby reduces the chemical pressure at R-sites which leads to a dramatic change in the crystal-field and molecular field at R-site. To explore the effect of chemical pressure, the experimental results of powder magnetic susceptibility and isothermal magnetization of a geometrically frustrated compound, Nd2Pb2O7 containing largest cation, e.g. lead (Pb), at M4+-sites are simulated and analyzed employing a D3d crystal-field (CF) and anisotropic molecular field at R-sites in the self-consistent mean-field approach. The second-ordered axial parameter B20 and total CF splitting of the ground multiplet 4I9/2 of Nd3+-ions became the lowest among the isomorphic Nd-pyrochlore compounds, implying reduced effect of the crystal-field at Nd sites. Nd2Pb2O7 has strong [111] Ising anisotropy. Relative strength and values of the exchange tensor among nearest-neighbor Nd3+-spins in Nd2Pb2O7 and Nd2Zr2O7 result in a very close competition of anti-ferromagnetic and ferromagnetic interactions.

Chemical pressure effect in Sm and La substituted ferroelectric BiFeO3 thin films: Insights from infrared spectroscopy

We investigate the effects of Sm and La substitution in ferroelectric BiFeO3 thin films on the lattice dynamics by infrared reflection measurements at room temperature. The frequencies of the infrared-active phonon modes are studied as a function of Sm and La content in Bi1–x(Sm,La)xFeO3 composition spread films in the range from x = 0 up to x = 0.25, grown on SrTiO3 substrates by pulsed laser deposition. Substitution of the Bi3+ ions with small Sm3+ ions leads to the appearance of a new phase above x ≈ 0.09 coexisting with the ferroelectric BiFeO3 phase up to x ≈ 0.19. In contrast, for the substitution of Bi3+ ions with La3+ ions of similar size a continuous transition from the original BiFeO3 phase to a new phase takes place. In both cases, we assign the new phase to the paraelectric, orthorhombic phase. These findings are discussed in terms of the morphotropic phase boundary in Sm-doped BiFeO3 around x = 0.14 with a phase coexistence, which was suggested as the origin for enhanced piezoelectric properties.

New insight into magneto-structural phase transitions in layered TbMn2Ge2-based compounds

The Tb1−xYxMn2Ge2 series (x = 0, 0.1, 0.2) compounds are found to exhibit two magnetic phase transitions with decreasing temperature: from the paramagnetic state to the antiferromagnetic interlayer state at TNinter and from an antiferromagnetic interlayer structure to a collinear ferrimagnetic interlayer structure at TCinter. Compared with the slight change of TNinter (409 K, 410 K and 417 K for x = 0, 0.1 and 0.2 respectively), the replacement of Y for Tb leads to a significant decrease in TCinter from 97.5 K for x = 0 to 74.6 K for x = 0.2. The variation in TCinter can be ascribed to the combination of two effects: (1) chemical pressure and (2) magnetic dilution effect by Y substitution for Tb. Besides, a strong anisotropic magnet-volume effect has been detected around TCinter in all compounds with Δa/a = 0.125%, 0.124% and 0.130% for x = 0, 0.1 and 0.2, respectively while no obvious effect is detected along the c-axis. The maximum magnetic entropy change were found to be −ΔSmax = 9.1 J kg−1 K−1, 11.9 J kg−1 K−1 and 6.3 J kg−1 K−1 with a field change from 0 T to 5 T for x = 0, 0.1, 0.2 respectively.

Change in crystal structure and physical properties of the Multiferroics YMnO3 single crystals by Strong gravitational field

Many researchers have studied the multiferroicity of the hexagonal RMnO3 (R: rare-earth element) for both applications and fundamental studies. To investigate the relationship between the structure and physical properties of materials, some people apply the chemical pressure effect. The procedure of chemical pressure effect involves substituting rare-earth elements for ones which have a different ionic radius. Mashimo et al. have developed a high-temperature ultracentrifuge apparatus that can generate extended duration strong gravitational field in excess of 106 G under a wide range of temperatures (up to 500°C). Strong gravitational fields directly act on each atom as a different body force. This can cause the change in crystal structure. Thus, we subjected YMnO3 single crystal to strong gravity experiments (0.78×106 G, 400°C, 2 h) and investigated the resulting changes in the crystal structure and physical properties of the gravity sample. The single crystal four-circle X-ray diffraction measurements revealed the change in the nearest neighboring Mn-Mn and M-O bond distances. The temperature dependence of magnetic susceptibility by SQUID showed the change in the magnetic anisotropy of gravity sample.

Effect of negative chemical pressure on the prototypical itinerant magnet MnSi

The evolution of the magnetic and charge transport properties of itinerant magnetic metal MnSi with the substitution of Al and Ga on the Si site is investigated. We observe an increase in unit cell volume indicating that both Al and Ga substitutions create negative chemical pressure. There are substantial increases in the Curie temperature and the ordered moment demonstrating that the substitutions give the magnetism a more local character. The substitutions also increase the range of temperature and field where the skyrmion phase is stable due to a change in the character of the magnetism. In contrast to the behavior of pure MnSi and expectations for the intrinsic anomalous Hall effect, we find a significant temperature dependence to the magnitude and sign of anomalous Hall conductivity constant in Al or Ga substituted samples. This temperature dependence likely reflects changes in the spin-orbit coupling strength with temperature, which may have significant consequences on the helical and skyrmion states. Overall, we observe a continuous evolution of magnetic and charge transport properties through positive to negative pressure

Anion substitution in hydrogen-bonded organic conductors: the chemical pressure effect on hydrogen-bond-mediated phase transition

We have synthesized three kinds of novel hydrogen-bonded (H-bonded) organic conductor, β′-[H3(Cat-EDO-TTF)2]X (X = ClO4, PF6, AsF6; Cat-EDO-TTF = catechol-fused ethylenedioxytetrathiafulvalene), as anion-substituted analogues of the parent BF4 salt (X = BF4). These salts are all isostructural, however, only the BF4 salt shows a phase transition, upon which the molecular arrangement and physical properties are drastically changed in cooperation with bending of the H-bond. A systematic comparison of their crystal and electronic structures disclosed that a significant chemical pressure is generated by the present anion substitution, causing differences in their physical properties and phase transition nature. Interestingly, this chemical pressure is highly anisotropic, which exclusively changes the side-by-side interactions between the Cat-EDO-TTF skeletons in the π-stacking layer. Therefore, we conclude that this unique phase transition is caused by a cooperation of the intercolumnar side-by-side interactions with the π–π intra-dimer interactions and H-bond bending.

Effect of high pressure on the topography of potential energy surfaces

Properties and reactivity of chemical compounds change dramatically at elevated pressures. Since kinetics and mechanisms of condensed-phase reactions are described in terms of their potential energy (PESs) or Gibbs energy (GESs) surfaces, chemical effects of high pressure can be assessed through analysis of pressure-induced deformations of GESs of solvated reaction systems. We discuss general trends expected for such changes and use quantum mechanical calculations to construct PESs of compressed species for hydrogen and methyl transfer reactions.

Structure evolution upon chemical and physical pressure in (Sr1−xBax)2FeSbO6

Here we demonstrate the gradual structural transformation from the monoclinic I2/m to tetragonal I4/m, cubic Fm-3m and hexagonal P63/mmc structure upon the isovalent larger-for-smaller A-site cation substitution in the B-site ordered double-perovskite system (Sr1−xBax)2FeSbO6. This is the same transformation sequence previously observed up to Fm-3m upon heating the parent Sr2FeSbO6 phase to high temperatures. High-pressure treatment, on the other hand, transforms the hexagonal P63/mmc structure of the other end member Ba2FeSbO6 back to the cubic Fm-3m structure. Hence we may conclude that chemical pressure, physical pressure and decreasing temperature all work towards the same direction in the (Sr1−xBax)2FeSbO6 system. Also shown is that with increasing Ba-for-Sr substitution level, i.e. with decreasing chemical pressure effect, the degree-of-order among the B-site cations, Fe and Sb, decreases.